1. Field of the Invention
The invention relates to a moving target indication radar and, more particularly, a radar of this kind capable of effectively rejecting undesirable returns (clutter) caused particularly by rain fall, sea surface and the like.
2. Prior Art
A radar system, particularly an air traffic control (ATC) radar, must discriminate the returns from moving objects (targets) from those undesirable returns from the stationary objects such as buildings and hills.
The undesirable returns include ground clutter attributed to buildings and undulating terrains, sea clutter caused by sea surface, weather clutter caused by rainfall and rain clouds, angel echo arising from large flocks of migrating birds, and the like. The conventional MTI (moving target indicator) is well adapted for rejecting the ground clutter among those clutters. The MTI canceller, however, is unable to reject a clutter having a speed component such as sea clutter, angel echo, weather clutter and the like. The dual-beam technique designed to reduce the receiver gain for low angle incidence returns at a close range is used for sea clutter and angel echo thereby to improve the S/C ratio, the ratio of the target return to the clutter. The circular polarized wave converter is used for weather clutter for the same purpose. Since those clutters have various levels, the receiver tends to be saturated, with a result that the target return is buried in those clutters making it impossible to detect or discriminate the target returns.
LOG-CFAR (Logarithmic Amplification and Constant False Alarm Rate) technique, which has been proposed to alleviate these disadvantages, is discussed in detail in a paper entitled "Detection Performance of the Cell Averaging LOG/CFAR Receiver" by V. G. Hansen and H. R. Ward, IEEE Transactions on AES, AES-8, No. 5, pp. 648-652, 1972. The LOG/CFAR technique, based on the fact that the sea and weather clutters each have an amplitude distribution similar to the Rayleigh distribution, employs the combination of a logarithmic amplifier and a CFAR circuit (including an average value measuring circuit and an anti-logarithmic converter) to suppress the clutter components to a level comparable to the noise level inherent in the radar receiver, and to optimize the threshold level with respect to the level suppressed, whereby the probablility that the clutters are detected erroneously as the target returns, called false-alarm rate, is rendered constant.
However, the LOG/CFAR technique is effective only when the amplitude distribution of the clutter signal is analogous to the Rayleigh distribution. Accordingly, it fails to attain the constant-false-alarm rate (CFAR) for clutter signals with other amplitude distributions resulting in the increase of clutter residue.
As stated above, the LOG/CFAR technique involves the problem of clutter residue. Recent observations show that clutters with amplitude distribution analogous to the Rayleigh distribution are rare, and that most of the clutters have the Weibull distribution. This is discussed in a paper entitled "Radar Detection in Weibull Clutter" by D. C. Schleher, IEEE Transaction on AES, AES-12, No. 6, pp. 736-743 (1976).
A proposal has been made for a technique to attain a constant false-alarm rate in Weibull clutter, by V. Gregers Hansen in his paper entitled "Constant False Alarm Rate Processing in Search Radars" presented at the International Conference on RADAR-PRESENT AND FUTURE, 23-25 October 1973.
This technique is capable of obtaining a constant false-alarm rate if the cumulative density function of a radar return signal is known even if the return signal has any type of time-domain distribution of amplitudes including the Weibull distribution. To describe in brief, when the cumulative density function q(x) of an input signal x is known, the input signal is subjected to a specific variable transformation (Z=log[1-q(x)]) which is determined by the cumulative density function so that the distribution of the input signal is transformed into an exponential distribution (P.sub.E (Z)=exp (Z)) whose parameters are determined by such distribution characteristics of the input signal as the average value, the variance and the energy and are constant.
Although the above-described technique may attain the constant false-alarm rate even for clutter with the Weibull distribution, it is still not completely satisfactory, particularly when processing clutters having the Rayleigh distribution. Since the Rayleigh distribution is a special case of the Weibull distribution, theoretically, the processing method for the Weibull clutter must be applicable to clutters of Rayleigh distribution. Practically, however, when applied to the processing of the Rayleigh clutters, it is inferior to the conventional LOG/CFAR technique. This technique requires the variable conversion of the input signal and, accordingly, additional circuits therefor. Thus, the conventional technique stated above involves complicated hardware and an increase in errors arising from the variable conversion processing.
Another proposal to attain the constant false-alarm rate for Weibull clutters has been made by G. B. GOLDSTEIN in his paper entitled "False-Alarm Regulation in Log-Normal and Weibull Clutter" published in the IEEE Transactions on AES, VOL. AES-9, No. 1, pp. 84-92 (January 1973). The proposed technique is applicable only to a specified Weibull clutter having two unique parameters defining the amplitude distribution. In the proposed technique, a threshold value is calculated to attain the constant false-alarm rate based on the comparison of the threshold value and a signal resulting from the logarithmic conversion of the input signal.
Furthermore, since the threshold value is calculated only for a specific Weibull clutter as stated above, it fails to attain the constant false-alarm rate for a general Weibull clutter with different parameters. Therefore, for this technique to be applied to the general Weibull clutter, the following processing will have to be made although not referred to in the paper. The estimated values of the parameters must be set in advance to allow the threshold values for all the combinations of these parameter values to be calculated and then stored in a memory such as a read only memory (ROM). Two parameters to define the Weibull distribution must then be measured from an input signal to enable the threshold value to be calculated on the basis of the measured parameters.
However, this requires a ROM memory of large capacity and a great number of processing steps, complicating the circuit structures.